1. Field of the Invention
This invention relates to a method and apparatus which produces a layer of material on a substrate by extracting ions from a laser ablation plume in a vacuum environment. In one embodiment, the invention relates to a method for producing diamond-like carbon films of nearly uniform thickness, surface smoothness approaching one angstrom, and a uniform index refraction approaching pure diamond (about 2.4).
2. Description of the Related Art
In recent years, there has been great interest in producing diamond-like carbon coatings for a variety of reasons. First, diamond-like carbon is an extremely hard surface nearly impervious to physical abuse (abrasive or chemical) and is therefore quite useful as a protective surface. Diamond-like carbon is optically transparent (in e.g., the infrared spectrum), and is therefore believed to be useful in a variety of optics applications such as protecting sensor optics optical circuits, quantum wells, etc. In addition, diamond-like carbon has been found to have a high electrical resistance as well as high thermal conductivity--an unusual combination. Thus, it is postulated that diamond-like carbon films would be quite useful in the electronics industry as a protective, resistive coating with extremely desirable heat sink properties. Diamond-like carbon when doped, can act as a semiconductor itself, forming the basis of technology for microcircuitry which can operate under high hostile conditions of high temperatures and radiation levels. Therefore, there has been great interest in developing techniques for obtaining diamond-like carbon films in commercial quantities for possible use in the semiconductor industry.
Currently, there are four major methods being investigated for producing diamond-like carbon films: (1) ion beam deposition; (2) chemical vapor deposition; (3) plasma enhanced chemical vapor depositions; and (4) sputter deposition. The ion beam deposition method typically involves producing carbon ions by heating a filament and accelerating carbon ions to selected energies for deposit on a substrate or high vacuum environment. Ion beam systems use differential pumping and mass separation techniques to reduce the level of impurities in the carbon ion fluence a rowing film. While acceptable films of diamond-like carbon can be obtained, the films are expensive to produce and are only achievable at very slow rates of growth on the order of 50 angstroms per day.
The chemical vapor deposition and plasma enhanced chemical vapor deposition methods are similar in operation and associated problems. Both methods use the dissociation of organic vapors such as CH.sub.3 OH, C.sub.2 H.sub.2, and CH.sub.3 OHCH.sub.3 to produce both carbon ions and neutral atoms of carbon for deposit on a substrate. Unfortunately, the collateral products of dissociation frequently contaminate the growing film. While both chemical vapor deposition and plasma enhanced chemical vapor deposition achieve film growth rates of practical levels, such films are of poor optical quality and unsuitable for most commercial uses.
Sputtering deposition usually includes two ion sources, one for sputtering carbon from a graphite source onto a substrate, and another ion source for breaking the unwanted graphite bonds in the growing film. For example, an argon ion sputtering gun sputters pure carbon atoms off of a graphite target within a vacuum chamber, and the carbon atoms are condensed onto a substrate. At the same time, another argon ion source cobombards the substrate to enhance the breakdown of the graphite bonding in favor of a diamond-like or tetrahedral bond in the growing carbon film. The high vacuum pressure (10.sup.-5 to 10.sup.-4 torr) in sputtering deposition is cumbersome and tends to introduce contamination of the film on a level comparable to those encountered in chemical vapor deposition and plasma enhanced chemical vapor deposition.
Therefore, while many attempts have been made to obtain high quality diamond-like carbon at commercial levels of production, the results have thus far been disappointing. The known methods recited above are deficient in many respects. While the ion beam deposition method produces acceptable quality film, its slow growth rates are impractical. The chemical vapor deposition and sputter methods are prone to contamination yielding an unacceptable film in most circumstances. All known methods require elevated temperatures, which often prove impractical if coating an optical substrate is desired. The known methods all involve complex and cumbersome devices to implement.
Producing diamond-like carbon is just one example of the general problem of producing a layer of material having desirable physical properties where the material is extremely difficult to handle or manipulate. Examples of other such materials include semiconductors, such as silicon, germanium, gallium arsenide, and recently discovered superconducting materials which might be generally characterized as difficult to handle ceramics (e.g., yttrium-barrium compounds). Therefore, it would be a significant advance to achieve a method and apparatus which could produce a high quality diamond-like carbon layer in commercial quantities. Further, it would be significant if such method and apparatus were useful in producing layers of other types of materials which using conventional technology are difficult to handle or produce.